1. Field of the Invention
Present invention relates to a wire having a flux core used for zinc-based alloy coated steel sheet. The wire having a flux core provides a weld portion which has no weld crack and is excellent in corrosion resistance without post-treatment such as touch-up, and in ductility and welding workability.
2. Description of the Related Art
Zinc coated steel sheets are widely used in fields such as building and automobile industries in view of improvement of corrosion resistance of structural members. Conventionally to improve the corrosion resistance, a method is used where an uncoated steel member is welded and then is coated by dipping the welded steel member in a zinc-based alloy bath. However, in this method, since the plating process is carried out after the welding step, productivity is low and equipment such as a plating bath is needed, which causes increased manufacturing costs. To avoid this, a method has been employed wherein steel sheets coated with zinc are welded to build a structure. Recently for further improvement of corrosion resistance of a structural member, a zinc-based alloy coated steel sheet is welded to build a weld structure. The zinc-based alloy coated steel sheet is made for example by plating the surface of steel sheet with Zn—Al—Mg—Si alloy having high corrosion resistance. (For example, see JP2000-064061 A)
As a specific problem caused by the method where a zinc coated steel sheet is welded to build a weld structure, it has been known that a liquid metal embrittlement crack results from melted plating material often occurring at the location of a weld metal and a welded heat affected zone (hereinafter referred to as “zinc embrittlement crack”). It is thought that the zinc embrittlement crack is mainly caused by the fact that melted zinc-based alloy plating components stay on the surface of a welded heat affected zone near the weld portion and break into the crystal grain boundary of the weld portion. It is also thought that a zinc plating material on the surface of weld portion would not be a cause of zinc embrittlement crack since the zinc plating material is evaporated during welding.
As for welding of a stainless steel structure which has needed higher corrosion resistance, same-metal-based welding materials of stainless steel are used. Weld metals of stainless steel formed at joint portions between stainless steels or between stainless steel and carbon steel has a good corrosion resistance as well as stainless steel.
However, according to the results of the present inventors' experiments, it was confirmed that even when using 309 type or 329 type stainless welding materials for obtaining a weld metal excellent in corrosion resistance with respect to welding a zinc coated steel sheet, there were a lot of zinc embrittlement cracks. That is, use of stainless steel welding material does not work with respect to welding of zinc coated steel.
In order to solve the problem of zinc embrittlement cracks, the inventors proposed a wire having a flux core which prevents a weld metal from having zinc embrittlement crack through controlling an area percentage of ferrite structure and tensile strength of the weld metal by adjusting contents of C, Si, Mn, Ni, Cr, and further content of TiO2 in the slag of the wire to an appropriate value. (For example as shown in JP2006-035293 A).
However zinc embrittlement cracks still often occur in using the above-mentioned wire having a flux core depending on the welding conditions, that is, prevention of zinc embrittlement cracks is not consistent. Also an obtained weld metal has low ductility, poor slag-separability and low arc-stability in welding work.
The inventors diligently studied about a joint preventing zinc embrittlement crack, and disclosed the result in WO2007/037447.